This invention relates to drive transmissions for transmitting torque from an engine or the like to mechanism which is driven thereby and more particular to planetary gearing systems providing for selective reversal of the drive direction.
Drive transmissions, as used for transmitting torque from the engine to the wheels of a vehicle, for example, typically have a series of internal gears which may be divided into two sections in terms of function, although there may not be a physical separation between the two sections. One section of the total gear system, often termed the range transmission, functions to enable selection of a desired drive ratio. Typically, low, intermediate, high and direct drive speeds may be provided for by the range section of a transmission. The other section may be termed the directional section and may be connected in tandem with the range section to provide for selection of either forward or reverse drive.
In transmissions of the particular class which utilize planetary gearing, reversal of drive direction can be accomplished by utilizing clutches or brakes to lock up all components of a planetary gearing set against relative rotation thereby providing for drive in a first direction while drive in an opposite direction can be provided for by driving a sun or a ring gear and taking drive from the other while braking the planetary carrier. While this is satisfactory for some transmissions, it suffers from the disadvantage that the drive in one direction is at a one-to-one ratio while the drive in the opposite direction is at some other ratio determined by the relative sizes of the components of the planetary gearing set.
In some transmission usages, such as in some forms of earth moving vehicle for example, it is desirable that the same sequence of speed ranges be provided in either forward or reverse direction. This cannot be accomplished with a single conventional planetary gear set of the kind described above. To realize such an objective, it is necessary to add additional gears and other components to the directional section of the transmission. This has undesirable effects in the form of added bulk, weight, cost, and the added inertial load in the power path.